Most electronic chips are built by layering different materials on top of each other, with the layers disposed on a semiconductor wafer (typically silicon). As each new layer is added, a polishing or grinding step is often needed to remove excess layer material, to planarized the wafer (make it very flat), or to accomplish other goals. The polishing process is often referred to as chemical mechanical plana*rization (CMP). When a plurality of layers is required then a large number of CMP steps may be necessary. In addition, the chip building process often requires that very thin layers of material be removed evenly from a wafer. To ensure that the correct amount of material is removed at each CMP step, some means for determining when to end polishing is needed.
One such means is to use an optical sensor that senses how much layer material has been removed or senses when a new layer has been reached. However, using an optical sensor can be difficult since the sensor is disposed very near the wafer surface. In addition, a caustic slurry used during the CMP process may damage the sensor. Nevertheless, a number of ways exist to deploy the optical sensor such that it can take the necessary measurements of the wafer.
A number of designs for a window installed in a polishing pad are shown in Birang et al., Forming a Transparent Window in a Polishing Pad for a Chemical Mechanical Polishing Apparatus, U.S. Pat. No. 5,893,796 (Apr. 13, 1999). The wafer to be polished is on top of the polishing pad, and the polishing pad rests upon a rigid platen so that the polishing occurs on the lower surface of the wafer. That surface is monitored during the polishing process by an interferometer that is located below the rigid platen. The interferometer directs a laser beam upward, and in order for it to reach the lower surface of the wafer, it must pass through an aperture in the platen and then continue upward through the polishing pad. To prevent the accumulation of slurry above the aperture in the platen, a window is provided in the polishing pad. Regardless of how the window is formed, it is clear that the interferometer sensor is always located below the platen and is never located in the polishing pad.
Another method is shown in Schultz, Method and Apparatus for Mechanical Planarization and Endpoint Detection of a Semiconductor Wafer, U.S. Pat. No. 5,081,796 (Jan. 21, 1992). Schultz describes a method in which, after partial polishing, the wafer is moved to a position in which part of the wafer overhangs the edge of the platen. The wear on this overhanging part is measured by interferometry to determine whether the polishing process should be continued.
Optical sensors disposed within polishing pads are capable of performing the required layer analysis with high efficiency. It is possible to increase the uniformity of polishing of these pads by providing an optical assembly that is capable of moving up and down within the pad as the pad wears.